Cambridge Dictionary

*The Cambridge Dictionary is a British dictionary, but it lists both the British and American pronunciaton and spelling. Every day there is a new ‘word of the day’:
http://www.dictionary.cambridge.org/wordoftheday.asp
Each ‘word of the day’ is followed by the word used in a sentence.

If you are on twitter, they tweet the word of the day:
http://twitter.com/CambridgeWords

Merriam-Webster’s Learner’s Dictionary:

*Merriam-Webster is an American dictionary, but it lists both American and British pronunciation. Every day there is a new ‘word of the day’:
http://www.learnersdictionary.com/lwod.php
(If you click on the red speaker icon, you can hear someone say the word.)

The Merriam-Webster Lexicographer also has a twitter profile. He often tweets the most looked-up words of the day:
http://twitter.com/PeterSokolowski

The Word Guy

*On twitter, this person often tweets groups of related words with their definition and etymology (the origin of the word):

http://twitter.com/thewordguy